Sudoku technique guide

Box-Line Reduction Sudoku Technique

Box-line reduction happens when a row or column forces a candidate into one 3x3 box. This guide explains the idea in plain English, when to look for it, and the common mistake to avoid.

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At a glance

Difficulty

Beginner to Medium

This is where Box-Line Reduction normally sits in a human solving path.

Family

Locked candidate techniques

It belongs to the locked candidate techniques group of Sudoku logic.

Main action

Usually remove candidates

Most technique moves reduce notes first. That often reveals a simpler placement afterwards.

What is Box-Line Reduction?

Box-line reduction happens when a row or column forces a candidate into one 3x3 box.

The important thing is that this is logic, not guessing. You are using the current candidates to prove that a number must either go in one place, or cannot stay in another place.

In SudoSketch, this technique is designed to work with notes and Coach highlighting, so the key cells and removal cells can be shown clearly.

How Box-Line Reduction works

Look along one row or column.
Find a candidate that only appears inside one 3x3 box.
Remove that candidate from the other cells in that 3x3 box.

When to look for it

Look for Box-Line Reduction after easier moves have stopped working. First check for naked singles, hidden singles, locked candidates and simple pairs. If those do not move the puzzle forward, this technique may be worth checking.

For learning, do not try to scan for every advanced technique at once. Pick one method, understand the shape, and practise spotting that one pattern.

Common mistake

Make sure the candidate is locked to one box from the row or column before removing it inside the box.

If you are unsure, rebuild the candidates first. Bad notes create bad logic, and Sudoku will absolutely punish you for it like a tiny spreadsheet goblin.

Box-Line Reduction FAQ

What is Box-Line Reduction in Sudoku?